In 1998, a demoralised and fragmented Congress party was going through a turbulent phase and ruling just four states of Madhya Pradesh, Odisha, Mizoram and Nagaland. That's when Sonia Gandhi, now 66, took over and in six years restored the party's central place in the country's
polity.

After three consecutive defeats in 1996, 1998 and 1999, Gandhi was instrumental in bringing the party back to power in 2004 and since then the Congress has successfully led a coalition government at the Centre and is in power in 13 states.

On March 14, when she completes 15 uninterrupted years - a unique distinction - as the Congress president, Sonia Gandhi has put her son Rahul Gandhi firmly in the saddle with his anointment as the party vice-president.

"Congress was in disarray when Sonia Gandhi took over. She has been able to establish unity and authority," said Prof Zoya Hasan of Jawaharlal Nehru University.

Gandhi has also been credited for taking the Congress to the left-of-the-centre position and for crafting "good" coalitions.

"Her legacy is bringing Congress back to secular position with focus on welfare and distribution that helped the party to regain its dominance in the national polity in 2004," Hasan added.

After the assassination of her husband and former Prime Minister Rajiv Gandhi in 1991, Sonia Gandhi had repeatedly turned down appeals to join the Congress till 1997 when she finally agreed to enter politics.

By March 14, 1998, she was elected as the Congress president. The biggest challenge for her then was to rejuvenate the party that had crumbled and split over the years.

Initially, electoral success eluded her but she refused to give up. "Hamein BJP aur inke saathiyon ko dhvast karna hai (we have to demolish BJP and its allies)," she had said, launching her campaign for the 2004 Lok Sabha elections.

The Congress emerged as the single largest party and along with Left, and other "like minded parties" formed the UPA government.

Unperturbed by the foreign origin issue raked by her opponents, Gandhi had succeeded in "demolishing" NDA's "feel good" campaigns with her "aam aadmi" slogan.

And then she stunned the world by declining to take up the Prime Minister's post. "Power in itself has never attracted me, nor has position been my goal," she had then said.

The renunciation of power -- a move many described as one of her biggest political masterstrokes-- solidified Gandhi's position.

Under her leadership, the UPA retained power in 2009 general elections with the Congress winning 206 seats, the highest by any party since 1991.

But, Sonia Gandhi has faced criticism for delaying the process of restructuring the Congress organization.

Political analysts say the next big challenge for her is to ensure Congress' victory in coming assembly and Lok Sabha elections, which comes at a time when the party has been at the receiving end of several controversies.

"The middle class, in particular, is disenchanted with Congress because of corruption and inflation. The ongoing transition phase might also adversely impact Congress fortunes in coming elections. And in such a situation Gandhi will have to play a very important and central role," Hasan said.